1. Field of the Invention
The present invention relates to the field of optical memories. More particularly, the present invention relates to a method and a system for a content-addressable optical data storage system.
2. Description of the Related Art
Both precision and similarity searches are performed in many database applications. A precision search is an identification of all records in a database that exactly match a query argument. Typically, each record has a small number of dimensions, i.e., 5-20, that are defined with high resolution. A precision search is usually based on a combination of a few dimensions, and is desired to be relatively high speed. A similarity search is an identification of all records in a database that are similar to a query argument. For a similarity search, records typically have a large number of dimensions, i.e., 100-1000, that are defined with low resolution. A similarity search is usually based on most or all dimensions, with the degree of similarity as a desired output characteristic. Search speed is not necessarily an issue.
Algorithmic and optical correlation approaches have been developed for performing database searches. Algorithmic approaches exhibit a trade off between flexibility and speed by indexing responses to possible searches. Even though the search time through N records can be made less than O(N) for an algorithmic approach, the entire database must be re-indexed when a single record is added to the database.
Volume holography-based optical correlation has been proposed as a technique for searching for digital data that has been organized into a spatial pattern and then used as a template and query image. See, for example, B. J. Goertzen et al., Error-correcting code for volume holographic storage of a relational database, Optics Letters, 20(15), pp. 1655-1657, 1995. Holographic correlation is well-suited for searching databases that have records composed of fixed-length fields, but not for locating words at arbitrary locations within a page of text. Consequently, no serious demonstration of volume holography-based optical correlation has been shown.
A main focus for volume holography-based optical correlation thus far has been on forming smart-pixel arrays for detecting a patterned data page as readout by a reference beam. See, for example, B. J. Goertzen et al., Volume holographic storage for large relational databases, Optical Engineering, 35(7), pp. 1847-1853, 1995; R. D Snyder et al., Database filter: optoelectronic design and implementation, Applied Optics, 36(20), pp. 4881-4889, 1997; and K. G. Richling et al., Holographic storage and associative processing of analog and digital data, Proceedings of the 1997 IEEE LEOS Annual Meeting, pp. 132-133, 1997.
Optical correlation approaches using volume holography are based on a two-dimensional (2-D) cross-correlation between two images at a hardware level, such as disclosed by B. J. Goertzen et al., Volume holographic storage for large relational databases, Optical Engineering, 35(7), pp. 1847-1853, 1995. Lenses are used for performing a 2-D Fourier transform by implementing a convolution and correlation by Fourier transforming two images, multiplying the images, and then performing a second Fourier transform on the resulting product of the images. A hologram stores the Fourier transform of the first image in space as a multiplicand for the second image, and to shift the output angle of the correlation so that the correlation can be distinguished from the transmission of the second image.
A volume hologram is stored within a photosensitive material by interfering a desired information-carrying light beam, referred to as an object beam, with a reference beam to form a hologram of the desired information. The desired information or records are displayed on a pixellated input device, such as a Spatial Light Modulator (SLM), organized as spatial patterns forming "images" that are modulated onto an input laser beam to form the object beam. The object beam is then directed into a thick photosensitive storage material, such as a photorefractive crystal or a photopolymer. The reference light beam, which is coherent with the object beam, is also directed into the photosensitive storage material so that it interferes with the object beam. Several thousand holograms can be superimposed in the same volume of photosensitive storage material.
After storage, the respective reference beams for all the holograms that are superimposed in the volume of the photosensitive storage material can be reconstructed by illuminating the volume with a new object beam having a selected interrogating spatial pattern. The amount of light diffracted into each respective reference beam is a measure of the cross-correlation between the interrogating spatial pattern and the originally-stored pattern. Many cross-correlations can be output in parallel because as many as several thousand holograms can be superimposed in the same volume of photosensitive storage material.
An exemplary content-addressable optical data storage system is disclosed by U.S. Pat. No. 5,319,629 to Henshaw et al. The Henshaw et al. storage system includes a multidimensional optical storage medium that stores a plurality of holographic images representative of data. A search argument having a target data field is coupled into the optical storage medium for extracting a set of address fields at which data fields in the storage medium that match the target data field of the search argument are stored. The storage medium is a spectral hole burning (SHB) material having an address field that includes a wavelength address and a Bragg angle address. The wavelength address specifies a wavelength of light at which a particular data field is stored in the optical medium. The Bragg angle address specifies an angle of incidence of a reference light beam at which a particular data field is stored in the optical medium.
Optical correlation has also been used as a technique for pattern recognition for analog images, such as by detecting targets within a satellite photograph. The number of target templates that a real-time input image can be simultaneously compared against can be increased by multiplexing multiple volume holograms within a thick photosensitive storage material. Performance issues include signal detection within noise and clutter, such as non-target images in a satellite photograph, and image distortions, such as scale or rotation.
What is needed is a way in which many storage volumes can be interrogated simultaneously, thereby increasing the parallelism with which an optical data storage system can be searched. What is also needed is improved detection of optically-performed correlations that is robust in the presence of noise. Further still, what is needed is a way for performing similarity searches on holographically stored digital records.